Work vehicles employ transmissions that are quite different from those in standard passenger cars. In work vehicles, the power to weight ratio is typically much more limited than it is in automobiles. This means that the engines powering work vehicles are usually running at or near capacity much of the time.
As a result, and particularly for ground engaging vehicles such as tractors towing plows, for example, shifting is critical. In work vehicles, shifting must occur rapidly and with very little delay from the time power is disengaged from the drive wheels to the time power is reengaged to the drive wheels. This rapid disengagement and re-engagement permits work vehicles to change gears much more rapidly than is possible in standard automobiles.
Work vehicles are also provided with gearboxes having much closer gear ratios. Unlike a typical automobile which may have four or even five forward speeds, work vehicles typically have 8, 10 or even 20 forward gears. With this many forward gears, work vehicles spend a great deal of time a shifting from one gear ratio to another to optimize vehicle speed and engine load.
Because of these unusual demands on work vehicle transmissions, they are uniquely designed to permit rapid gear shifting over a wide range of gears. First, they have numerous internal transmission shafts, each carrying several gears. In a common automobile transmission, for example, there may be three shafts with two gears on each shaft. In a work vehicle transmissions, in contrast, there may be five shafts with as many as 12 or 14 gears. This many shafts and this many gears are necessary to provide the number of gear ratios that work vehicle transmissions typically have.
With this many gears, it is practically impossible to shift gears by sliding gears into and out of engagement. This is the traditional manner in automotive gearboxes for changing gears. Yet this system require as a complex array of shift yokes that are disposed inside the transmission case and slide gears back and forth from engagement with one gear to engagement with another gear and even to engagement with no gear at all.
Providing space for sliding the gears and for the shift yokes necessary to slide the gears is prohibitive. Furthermore, even if a transmission could be arrayed with all of the mechanical shifting mechanisms necessary to slide these gears the speed of shifting would be extremely slow.
For this reason, work vehicle transmissions have developed along an entirely different line than those of automotive transmissions. Unlike automotive transmissions, which have a single large master clutch to disengage the drive train, and three or four shifting forks to shift the gears on their shafts, work vehicle transmissions put several small clutches inside the transmission disposed between shafts and gears and between gears themselves.
These transmissions are called “power shift” transmissions. A typical power shift transmission with 15 forward speeds and 6 reverse speeds may have five internal clutches inside the transmission. Again, these clutches are disposed between internal transmission shafts and the gears that turn on the shafts and between adjacent gears themselves.
Clutches in power shift transmissions are “wet” clutches. In other words, they are designed intentionally to run soaked with oil. Indeed, unlike automotive dry plate master clutches, the only way to make wet clutches work in power shift transmissions is to ensure that they have a continuing supply of oil flooding them as they operate.
Oil is critical for the proper operation of wet clutches due to the extreme loads on the clutches in their compact size. Since they are so small, and since they engage and disengage with such great frequency, they are particularly prone to overheating, and are specifically designed with internal oil passages that conduct cooling transmission fluid between them through the plates themselves to conduct the heat they generate away from the clutches and (typically) into a transmission oil cooler.
Wet clutches are designed not with a single plate, but with multiple interdigitated or interleaved plates. These plates are typically much smaller and outside diameter than the single dry plate of an automotive clutch. They must be smaller in outside diameter to fit inside a transmission case without abutting adjacent transmission shafts.
Wet clutches typically have some 15 interdigitated plates. Half of these plates are typically engaged to rotate with the shaft on which the clutch is mounted, and the other half of the plates are typically engaged to rotate with a gear supported on that shaft and that spins freely with respect to that shaft until the clutch is engaged.
Heat is produced at every plate-to-plate junction. A typical stack of plates might be three or 4 inches tall and 5 inches in diameter with a plate width of 1 to 2 inches. To cool all of these junctions, special oil passages are typically drilled into the shaft on which they spin. Oil is forced through the shaft and out through radial holes that abut the inner surface of the stack of clutch plates. When oil is forced into the shaft of the transmission, it passes through this internal passageway down the length of the shaft, out the radial passageways, and into, through, and between the individual clutch plates. As the oil passes between each clutch plate, each clutch plate transfers its heat to the flowing oil. Once the oil passes through the clutch plates in the stack, it leaks out of the clutch entirely and into the open transmission case, where it falls to a common drain the and is pumped away to a transmission fluid cooler. Once it is cooled, it is again pumped back to the transmission, through the transmission shafts, and back through the plates.
As engines with greater and greater horsepower are installed in work vehicles, and as transmissions are made smaller and lighter, and as clutch plates are made more and more compact, more and more heat builds up rapidly in a work vehicle transmission. To remove this heat, engineers have had to redesign and reconfigure transmission clutches to ensure that all of the transmission fluid forced into the clutches in deed passes between the clutch plates and cools the clutch plates. It has recently been discovered that much of the oil passing from the shaft and against the inside surface of the clutch plates did not pass through the clutch plate. Instead of passing through the stack of clutch plates and cooling them, the oil bypasses the clutch plates, passing longitudinally down the clutch hub, between the clutch plates and the shaft, until it reached the end of the stack of clutch plates and ran out without cooling the clutch plates.
The last plate blocking the free flow of oil out of the clutch pack is the clutch backing plate. The clutch backing plate acts as the end cover to the clutch carrier. The clutch backing plate holds the stack of clutch plates inside the carrier. The clutch backing plate is engineered much thicker and stronger than each of the individual clutch plates that comprise the clutch stack.
The clutch backing plate in wet clutches typically has a thickness of between 0.1 and 0.3 inches in the axial direction of the clutch. This thickness gives the clutch backing plate the necessary strength to support one end of the stack of clutch plates when the plates are compressed together by a piston that is positioned in the clutch carrier at the opposite end of the clutch plate stack.
Today, all clutch backing plates are typically restrained in the clutch carrier by the same slots that the other clutch plates are restrained in. These slots extend longitudinally along the outer surface of the clutch carrier and engage small ears or protrusions on each clutch plate in the clutch stack. The engagement of the ears in the slots force the clutch plates to rotate with the clutch carrier when it rotates, yet permit the clutch plates to slide axially.
Since the clutch backing plate is supported in the same slots, it can also slide axially. This is normally not a problem. In the traditional design, the clutch backing plate generally stays at one end of the clutch carrier and does not exert any force on the clutch plate stack when the clutch piston is relaxed and retracted. With new changes to the clutch design, however, the free axial movement of the clutch backing plate in the slots that also support the clutch plate slack poses a problem.
To prevent oil from leaking out between the inner surface of the clutch plates and the clutch hub, engineers devised a system that includes an additional oil seal. This oil seal is disposed between and seals against both the clutch backing plate and the clutch hub. The additional oil seal, sealing against both the clutch hub and the clutch backing plate, prevented oil from flowing through this gap and out of the clutch. Since this exit was sealed off, oil was forced to flow through the clutch stack itself, cooling the plates of the clutch stack, and exiting through the longitudinal slots in the clutch carrier. This additional oil seal prevents the oil from leaking out of the clutch plate cavity and forces the oil to pass through and between the clutch plates. In that sense, it is a success. Unfortunately, it has caused other problems.
The new oil seal is preferably made of polytetrafluoroethylene (also known by its trade name as “Teflon” or PTFE). This PTFE seal swells slightly as the transmission fluid and the clutch components heat up. When it swells, it presses harder against the clutch backing plate. This additional pressure against the clutch backing plate (and the fact that the clutch backing plate typically has a rounded or chamfered edge) forces the clutch backing plate down the clutch carrier slots toward the clutch plate stack. On occasion, the expanded seal will actually push the clutch backing plate against the otherwise slack and disengaged clutch plates of the clutch plate stack.
The pressure of the clutch backing plate acting against the clutch plate stack is not enough to fully engage the clutch plates in the stack, locking them together, and inadvertently engaging the clutch. There is no safety issue. There is a wear issue, however the slight pressure provided by the clutch backing plate against the clutch plate stack is enough to force transmission fluid from between the plates and cause them to spin against each other. This causes the clutch plates to wear much faster than they normally would, and can cause heat damage to the clutch discs.
In addition, rapidly turning clutch plates may “flutter” or oscillate in an axial direction. This flutter affects the clutch backing plate and can cause the backing plate to press the clutch plates together, causing similar clutch plate wear and overheating problems.
What is needed, therefore, is a new clutch that both (1) seals the gap between the clutch backing plate and the shaft, and (2) reduces or eliminates the chance that the clutch backing plate will be pushed against the clutch plate stack.
What is also needed is a new clutch that limits the free movement of the clutch backing plate, thereby preventing it from inadvertently compressing the clutch plate stack.
What is also needed is a clutch with a clutch carrier that restrains the free movement of the clutch backing plate.
These and other advantages are provided by the clutch described below.